Cranes are apparatuses which are intended for transferring loads, both in the open air and in closed environments. In manufacturing factories a crane is typically a bridge crane, movable along tracks by means of a bridge moving in the direction of the tracks and one or more trolleys moving along the bridge in the direction substantially perpendicular to the direction of the tracks, on which trolleys with one or more ropes, or corresponding hoisting means, such as belts and chains, are mounted. Although in an ideal situation each rope is vertical or almost vertical so that the load is directly under the trolley, in real life the ropes every now and then deviate from the ideal situation, and a need to know the actual rope angle, i.e. how much the rope has deviated, arises.
Several methods for finding out a deflection angle of a load are known. Typically they measure the deflection angle in an upward direction, i.e. from a lifting element, such as a hook, towards a trolley. For example, publication JP 9-156878 discloses a solution in which an optical fibre gyroscope is fitted to a rope suspension of a crane near the hook or other hoisting means to measure a shake angle value of a suspended load. Publication DE 10008235 discloses utilizing accelerometers, installed in the hook, to determine a deflection angle of a load by multiplying the output of an accelerometer by a correction value that corresponds to the reciprocal of the earth's acceleration. Publication DE 4238795 discloses that the hook may be equipped with a group of three accelerometers, or with a gyroscope or with an inclinometer, provided that the gyroscopes and inclinometers have appropriate accuracy to determine the deflection angle. An article by Yong-Seok Kim, Keum-Shik Hong, and Seung-Ki Sul, published in International Journal of Control, Automation, and Systems, vol. 2, no. 4, pp. 435-449, December and having the title “Anti-Sway Control of Container Cranes: Inclinometer, Observer, and State Feedback”, suggests using, instead of a vision system, an inclinometer attached to a head block of a crane to detect a sway angle. A drawback of connecting a sensor in a hook to a controller in the crane requires either a long wiring, which is vulnerable to entanglement, or a wireless transmitter in the sensor and a corresponding receiver in the controller, and the powering of the sensor is also problematic. Another drawback is that a sensor located near the hook is rather vulnerable to external impacts, such as an accidental collision with the load when the load is attached to the hook.
WO 2009/138329 discloses a solution, which overcomes the above drawbacks since the measuring is performed downward. In the solution, when a load is transferred, a group of accelerometers, positioned on the rope in a part which is immobile or in the rope anchorage, calculates the displacement of a gripping element of the load in relation to a respective perpendicular Cartesian axis (x, y, z) by means of a rope deviation angle and a position of the hook in respect of the Z axis. The displacements on the three Cartesian axes of the hook of a lifting apparatus is described in WO 2009/138329 as an essential feature to the operations disclosed. A problem with the solution in WO 2009/138329 is that it ignores the fact that, especially when the rope deviation angle is measured by a sensor/sensors positioned in a vicinity of the rope anchorage, an acceleration or deceleration of the crane causes an error to the measured rope deviation angle.